Andrey Popp wrote:
> I think that producing a list of tuples (that is conceptually image of
> mapping from some_iterable set) is basic operation.
> But... ok, now we have three ways to produce list below:
>> [(f(x), f(x)) for x in some_iterable if f(x) < 2]
Taking in isolation, there is no reason to produce a list rather than an
iterator.
> 1)
>> def g(iterable):
>> for x in iterable:
>> y = f(x)
>> if y < 2:
>> yield (y, y)
This does more than the above because g is reusable both with the same
iterable and other iterables.
> 2)
>> [(y, y) for y in (f(x) for x in some_iterable) if y < 2]
Though I would probably write the reusable generator
I might write this as
ygen = (f(x) for x in some_iterable)
# or map(f, some_iterable) if f is an existing function
ypairs = ((y, y) for y in ygen if y < 2)
There are really two ideas:
map f to some_iterable
make pairs conditionally.
There should be no shame in putting each in a separate statement.
Nested generators do not really turn this into 'two' iterations, as
iterating with ypairs will run the implied loop in synchrony.
> 3)
>> map(lambda obj: (obj, obj),
>> filter(lambda y: y < 2,
>> map(f, some_iterable)))
>> And none of them does not look as obvious as
>> [(f(x), f(x)) for x in some_iterable if f(x) < 2]
>> , doesn't it? While proposed variant with where-clause
>> [(y, y) for x in some_iterable if y < 2 where y = f(x)]
>> looks more naturally than three suggested variants.
'natural' is in the eye of the beholder
> I give strong emphasis on that fact, that where-clause is only
> syntactic sugar, suggested for better readability.
Too much sugar = stomach ache ;-).
Terry Jan Reedy